U.S. patent number 9,294,022 [Application Number 13/940,769] was granted by the patent office on 2016-03-22 for bldc motor driving apparatus and refrigerator using the same.
This patent grant is currently assigned to SAMSUNG ELECTRONICS CO., LTD.. The grantee listed for this patent is Samsung Electronics Co., Ltd.. Invention is credited to Hyun Chang Cho, Koji Hamaoka, Seon Gu Lee, Pyeong Ki Park, Sung In Park, Ho Hyun Ryu, Jeong Ho Seo, Hyo Jea Shin, Takeda Yoshihiko.
United States Patent |
9,294,022 |
Park , et al. |
March 22, 2016 |
BLDC motor driving apparatus and refrigerator using the same
Abstract
A motor driving apparatus and a refrigerator using the same is
provided. The refrigerator may include a compressor, a motor, a
driving unit, temperature sensing units sensing the temperatures of
storage chambers and an external temperature, and a control unit
selecting a driving mode of the driving unit based on the sensing
result of the temperature sensing units and controlling the driving
unit to drive the motor according to the selected driving mode. In
a general operation mode, the control unit controls the driving
unit to drive the motor in a 120 degree conduction method, and in a
power-saving operation mode, the control unit controls the driving
unit to drive the motor in a 90 degree conduction method. The
refrigerator increases a pulse width of driving current by
converting the conduction method of the motor to drive the motor at
a low speed during power-saving operation of the refrigerator.
Inventors: |
Park; Sung In (Gwangju,
KR), Park; Pyeong Ki (Gwangju, KR), Seo;
Jeong Ho (Gwangju, KR), Shin; Hyo Jea (Gwangju,
KR), Ryu; Ho Hyun (Gwangju, KR), Lee; Seon
Gu (Gwangju, KR), Cho; Hyun Chang (Gwangju,
KR), Yoshihiko; Takeda (Gwangju, KR),
Hamaoka; Koji (Gwangju, KR) |
Applicant: |
Name |
City |
State |
Country |
Type |
Samsung Electronics Co., Ltd. |
Suwon-si |
N/A |
KR |
|
|
Assignee: |
SAMSUNG ELECTRONICS CO., LTD.
(Suwon-Si, KR)
|
Family
ID: |
48792987 |
Appl.
No.: |
13/940,769 |
Filed: |
July 12, 2013 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20140013784 A1 |
Jan 16, 2014 |
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Foreign Application Priority Data
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Jul 13, 2012 [KR] |
|
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10-2012-0076959 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H02P
6/182 (20130101); H02P 6/15 (20160201); F25B
49/025 (20130101) |
Current International
Class: |
H02P
6/20 (20060101); H02P 6/18 (20060101); H02P
7/00 (20060101); H02P 6/08 (20060101) |
Field of
Search: |
;62/228.1
;318/400.34,700,434,400.32-400.35 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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|
|
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2008-160950 |
|
Jul 2008 |
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JP |
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10-0654813 |
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Dec 2006 |
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KR |
|
Primary Examiner: Ali; Mohammad M
Assistant Examiner: Shaikh; Meraj A
Attorney, Agent or Firm: Staas & Halsey LLP
Claims
What is claimed is:
1. A refrigerator comprising: a motor to provide rotating force to
a compressor; a driver to drive the motor and include a plurality
of switching elements; temperature sensors to sense the
temperatures of storage chambers; and a controller to select a
driving mode of the driver based on the sensing result of the
temperature sensors, and to control the driver to drive the motor
according to the selected driving mode, wherein, in a general
operation mode, the controller controls the driver to drive the
motor in a 120 degree conduction method, and in a power-saving
operation mode, the controller controls the driver to drive the
motor in a 90 degree conduction method, wherein, in the 120 degree
conduction method, a switching element having been activated, of
the plurality of switching elements is deactivated when the motor
is rotated by 120 degrees after the switching element is activated,
and wherein, in the 90 degree conduction method, a switching
element having been activated, of the plurality of switching
elements is deactivated when the motor is rotated by 90 degrees
after the switching element is activated.
2. The refrigerator according to claim 1, wherein the motor is a
brushless direct current (BLDC) motor, the refrigerator further
comprises a position sensor to sense the position of a rotor of the
BLDC motor, and the position to detect back electromotive force
generated from coils of the BLDC motor.
3. The refrigerator according to claim 2, wherein the controller
includes a mode selector to select the driving mode of the driver
based on the sensing result of the temperature sensors, a speed
controller to calculate a rotating speed of the BLDC motor based on
the output of the position sensor and to generate a speed control
signal controlling the rotating speed of the BLDC motor based on
the calculated rotating speed of the BLDC motor, a driving signal
generator to generate driving signals to control the driver based
on the output of the mode selector and the output of the speed
controller, and a pulse width modulator to modulate the pulse width
of the output of the driving signal generator.
4. The refrigerator according to claim 3, wherein the speed
controller calculates zero crossing points (ZCPs) based on the
sensing result of the position sensor, calculates the position of
the rotor of the BLDC motor based on the calculated ZCPs, and
calculates the rotating speed of the BLDC motor based on the
calculated position of the rotor.
5. The refrigerator according to claim 2, wherein the position
sensor includes a voltage divider provided between input terminals
of the BLDC motor and ground, and provides the output of the
voltage divider to the controller.
6. The refrigerator according to claim 2, wherein the driver
includes a rectifier circuit to rectify external power, a smoothing
circuit to smooth DC power rectified by the rectifier circuit, and
a driving circuit to generate driving current of the BLDC motor
based on the output of the controller.
7. A motor driving apparatus comprising: a driver to drive a motor
and include a plurality of switching elements; and a controller to
control the driver to drive the motor according to one of a 120
degree conduction method and a 90 degree conduction method,
wherein, in the 120 degree conduction method, a switching element
having been activated, of the plurality of switching elements is
deactivated when the motor is rotated by 120 degrees after the
switching element is activated, and wherein, in the 90 degree
conduction method, a switching element having been activated, of
the plurality of switching elements is deactivated when the motor
is rotated by 90 degrees after the switching element is
activated.
8. The motor driving apparatus according to claim 7, wherein the
motor is a brushless direct current (BLDC) motor, the motor driving
apparatus further comprises a position sensor to sense the position
of a rotor of the BLDC motor, and the position sensor detects back
electromotive force generated from coils of the BLDC motor.
9. The motor driving apparatus according to claim 8, wherein the
controller includes a mode selector to select the driving mode of
the driver according to the external signal, a speed controller to
calculate a rotating speed of the BLDC motor based on the output of
the position sensor and to generate a speed control signal to
control the rotating speed of the BLDC motor based on the
calculated rotating speed of the BLDC motor, a driving signal
generator to generate driving signals to control the driver based
on the output of the mode selector and the output of the speed
controller, and a pulse width modulator to modulate the pulse width
of the output of the driving signal generator.
10. The motor driving apparatus according to claim 9, wherein the
speed controller calculates zero crossing points (ZCPs) based on
the sensing result of the position sensor, calculates the position
of the rotor of the BLDC motor based on the calculated ZCPs, and
calculates the rotating speed of the BLDC motor based on the
calculated position of the rotor.
11. The motor driving apparatus according to claim 8, wherein the
position sensor includes a voltage divider provided between input
terminals of the BLDC motor and ground, and provides the output of
the voltage divider to the controller.
12. The motor driving apparatus according to claim 8, wherein the
driver includes a rectifier circuit to rectify external power, a
smoothing circuit to smooth DC power rectified by the rectifier
circuit, and a driving circuit to generate driving current of the
BLDC motor based on the output of the controller.
13. A motor driving apparatus comprising: a driver to drive a motor
and include a plurality of switching elements; and a controller to
control the driver to drive the motor according to a 90 degree
conduction method, wherein, in the 90 degree conduction method, a
switching element having been activated, of the plurality of
switching elements is deactivated when the motor is rotated by 90
degrees after the switching element is activated.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
This application claims the priority benefit of Korean Patent
Application No. 10-2012-0076959, filed on Jul. 13, 2012 in the
Korean Intellectual Property Office, the disclosure of which is
incorporated herein by reference.
BACKGROUND
1. Field
Embodiments relate to a brushless direct current (BLDC) motor
driving apparatus which achieves power-saving operation of a
refrigerator, and a refrigerator using the same.
2. Description of the Related Art
In general, a brushless direct current (BLDC) motor uses an
inverter circuit including switching elements instead of mechanical
elements, such as a brush and a commutator, and is characterized in
that replacement of a brush due to abrasion of the brush is not
required and electromagnetic interference and noise are
minimal.
Such a BLDC motor has been widely used in products requiring
high-frequency and variable-speed operation, such as a compressor
for refrigerators and air conditioners, and a washing machine.
In order to operate the BLDC motor, flux of a stator needs to be
controlled so as to have a right angle or a random angle to flux of
a permanent magnet generated from a rotor. For this purpose, the
position of the rotor is detected at all times, and the switching
state of the switching elements of the inverter is determined so
that a flux generating position of the stator is determined
according to the position of the rotor. Here, in order to detect
the position of the rotor, a resolver, an absolute encoder, or a
hall sensor may be used. In case of a compressor for refrigerators
and air conditioners, use of a sensor is difficult due to
environmental factors, such as temperature and pressure, and thus a
sensorless type in which the position of a rotor is detected from
voltage or current applied to a motor is mainly used. In general, a
sensorless BLDC motor detects the position of a rotor using back
electromotive force (back-EMF) of a motor detected through a
position detection circuit.
In order to control the rotating speed of the BLDC motor, a pulse
width modulation (PWM) method is generally used. That is, driving
current applied to the motor is adjusted by inputting driving
current, the pulse width of which is modulated, and the rotating
speed of the BLDC motor is controlled through the adjusted driving
current.
In order to reduce energy consumption, low-speed operation in which
a motor used in a compressor for refrigerators or air conditioners
is rotated at a low speed is required, and in order to achieve such
low-speed operation, the duty of a pulse needs to be decreased.
If the pulse duty of driving current applied to the BLDC motor is
very small, detection of the position of the rotor may be
difficult.
SUMMARY
In an aspect of one or more embodiments, there is provided a BLDC
motor driving apparatus and method in which a pulse duty of driving
current is increased during low-speed operation of a BLDC
motor.
In an aspect of one or more embodiments, there is provided a
refrigerator includes a compressor, a motor providing rotating
force to the compressor, a driving unit driving the motor,
temperature sensing units sensing the temperatures of storage
chambers and an external temperature, and a control unit selecting
a driving mode of the driving unit based on the sensing result of
the temperature sensing units, and controlling the driving unit to
drive the motor according to the selected driving mode, wherein, in
a general operation mode, the control unit controls the driving
unit to drive the motor in a 120 degree conduction method, and in a
power-saving operation mode, the control unit controls the driving
unit to drive the motor in a 90 degree conduction method.
The motor may be a BLDC motor, the refrigerator may further include
a position sensing unit sensing the position of a rotor of the BLDC
motor, and the position sensing unit may detect back electromotive
force generated from coils of the BLDC motor.
The control unit may include a mode selector selecting the driving
mode of the driving unit based on the sensing result of the
temperature sensing units, a speed controller calculating a
rotating speed of the BLDC motor based on the output of the
position sensing unit and generating a speed control signal
controlling the rotating speed of the BLDC motor based on the
calculated rotating speed of the BLDC motor, a driving signal
generator generating driving signals controlling the driving unit
based on the output of the mode selector and the output of the
speed controller, and a pulse width modulator modulating the pulse
width of the output of the driving signal generator.
The speed controller may calculate zero crossing points (ZCPs)
based on the sensing result of the position sensing unit, calculate
the position of the rotor of the BLDC motor based on the calculated
ZCPs, and calculate the rotating speed of the BLDC motor based on
the calculated position of the rotor.
The position sensing unit may include a voltage divider provided
between input terminals of the BLDC motor and ground and provide
the output of the voltage divider to the control unit.
The driving unit may include a rectifier circuit rectifying
external power, a smoothing circuit smoothing DC power rectified by
the rectifier circuit, and a driving circuit generating driving
current of the BLDC motor based on the output of the control
unit.
In an aspect of one or more embodiments, there is provided a motor
driving apparatus includes a driving unit driving a motor, and a
control unit selecting a driving mode of the driving unit according
to an external signal, and controlling the driving unit to drive
the motor according to the selected driving mode, wherein, in a
general operation mode, the control unit controls the driving unit
to drive the motor in a 120 degree conduction method, and in a
power-saving operation mode, the control unit controls the driving
unit to drive the motor in a 90 degree conduction method.
The motor may be a BLDC motor, the motor driving apparatus may
further include a position sensing unit sensing the position of a
rotor of the BLDC motor, and the position sensing unit may detect
back electromotive force generated from coils of the BLDC
motor.
The control unit may include a mode selector selecting the driving
mode of the driving unit according to the external signal, a speed
controller calculating a rotating speed of the BLDC motor based on
the output of the position sensing unit and generating a speed
control signal controlling the rotating speed of the BLDC motor
based on the calculated rotating speed of the BLDC motor, a driving
signal generator generating driving signals controlling the driving
unit based on the output of the mode selector and the output of the
speed controller, and a pulse width modulator modulating the pulse
width of the output of the driving signal generator.
The speed controller may calculate zero crossing points (ZCPs)
based on the sensing result of the position sensing unit, calculate
the position of the rotor of the BLDC motor based on the calculated
ZCPs, and calculate the rotating speed of the BLDC motor based on
the calculated position of the rotor.
The position sensing unit may include a voltage divider provided
between input terminals of the BLDC motor and ground and provide
the output of the voltage divider to the control unit.
The driving unit may include a rectifier circuit rectifying
external power, a smoothing circuit smoothing DC power rectified by
the rectifier circuit, and a driving circuit generating driving
current of the BLDC motor based on the output of the control
unit.
BRIEF DESCRIPTION OF THE DRAWINGS
These and/or other aspects will become apparent and more readily
appreciated from the following description of embodiments, taken in
conjunction with the accompanying drawings of which:
FIG. 1 is a front view briefly illustrating a refrigerator in
accordance with an embodiment;
FIG. 2 is a cross-sectional view briefly illustrating the structure
of a three-phase BLDC motor in accordance with an embodiment;
FIG. 3 is a view illustrating back electromotive force generated
from each coil of the three-phase BLDC motor in accordance with an
embodiment;
FIGS. 4A and 4B are views illustrating back electromotive force
generated from each coil and current flowing in each coil of the
three-phase BLDC motor in accordance with an embodiment;
FIG. 5 is a block diagram illustrating control flow of the
refrigerator in accordance with an embodiment;
FIG. 6 is a block diagram illustrating control flow of a compressor
motor driving apparatus driving a compressor in accordance with an
embodiment;
FIG. 7 is a circuit diagram briefly illustrating a compressor
driving unit and a position sensing unit in accordance with an
embodiment;
FIG. 8 is a block diagram illustrating control flow of a control
unit controlling the compressor in accordance with an
embodiment;
FIG. 9 is a block diagram illustrating a pulse width modulator of
the control unit in accordance with an embodiment;
FIG. 10 is a view illustrating output of the pulse width modulator
of the control unit in accordance with an embodiment;
FIG. 11 is a view illustrating flow of driving signals and driving
current driving the compressor motor if the refrigerator in
accordance with an embodiment is in a general operation mode;
FIG. 12 is a view illustrating flow of driving signals and driving
current driving the compressor motor if the refrigerator in
accordance with an embodiment is in a power-saving operation mode;
and
FIG. 13 is a view illustrating efficiency of the compressor motor
if the refrigerator in accordance with an embodiment is in the
power-saving operation mode and efficiency of a compressor motor of
a conventional refrigerator.
DETAILED DESCRIPTION
Reference will now be made in detail to embodiments, examples of
which are illustrated in the accompanying drawings, wherein like
reference numerals refer to like elements throughout.
FIG. 1 is a front view briefly illustrating a refrigerator 100 in
accordance with an embodiment.
With reference to FIG. 1, the refrigerator 100 in accordance with
an embodiment includes a main body 110 forming the external
appearance of the refrigerator 100, storage chambers 121 and 122
storing articles, a cooling device 161, 171, 181, 182, 191 and 192
cooling the storage chambers 121 and 122, and temperature sensing
units (temperature sensors) 141, 142 and 143 sensing temperatures
of the storage chambers 121 and 122.
The storage chambers 121 and 122 storing articles and ducts (not
shown) provided with evaporators 191 and 192 which will be
described later are provided in the main body 110, and holes (not
shown) through which air cooled by the evaporators 191 and 192
flows between the ducts (not shown) and the storage chambers 121
and 122 are provided on the wall surfaces of the main body 110
provided with the storage chambers 121 and 122.
The storage chambers 121 and 122 includes a freezing chamber 121
storing articles in a frozen state and a refrigerating chamber 122
storing articles in a refrigerated state which are divided side by
side by a diaphragm, and the front surfaces of the freezing chamber
121 and the refrigerating chamber 122 are opened.
The freezing chamber 121 and the refrigerating chamber 122 are
opened and closed by respective doors 131 and 132. An input unit
111 and a display unit 112 which will be described later may be
provided on the doors 131 and 132 of the refrigerator 100.
The temperature sensing units 141, 142 and 143 sensing temperatures
of the storage chambers 121 and 122 are provided in the storage
chambers 121 and 122, and include a first temperature sensing unit
141 sensing the temperature of the freezing chamber 121 and a
second temperature sensing unit 142 sensing the temperature of the
refrigerating chamber 122. The temperature sensing units 141, 142
and 143 may further include an external temperature sensing unit
143 (FIG. 5) provided at the outside of the refrigerator 100 and
sensing the temperature of the outside of the refrigerator 100.
The temperature sensing units 141, 142 and 143 may employ
thermistors, electrical resistance of which is varied according to
temperature.
Cooling fans 151 and 152 cause air cooled by the evaporators 191
and 192 provided in the ducts (not shown) to flow into the storage
chambers 121 and 122.
The cooling device 161, 171, 181, 182, 191 and 192 includes a
condenser 171 condensing a refrigerant in a vapor phase, expansion
valves 181 and 182 decompressing the condensed refrigerant in a
liquid phase, evaporators 191 and 192 evaporating the decompressed
refrigerant in the liquid phase, and a compressor 161 compressing
the evaporated refrigerant in the vapor phase. In the evaporators
191 and 192, the refrigerant is converted from the liquid phase to
the vapor phase, and during such a process, the refrigerant absorbs
latent heat and thus cools the evaporators 191 and 192 and air
around the evaporators 191 and 192.
The condenser 171 may be installed in a machine chamber (not shown)
provided in the lower portion of the main body 110, or be installed
at the outside of the main body 110, i.e., on the rear surface of
the refrigerator 100. The refrigerant in the vapor phase is
condensed into the liquid phase through the condenser 171. During
such a condensing process, the refrigerant discharges latent
heat.
If the condenser 171 is installed in the machine chamber provided
in the lower portion of the main body 110, the condenser 171 is
heated by latent heat discharged from the refrigerant, and thus a
radiation fan (not shown) cooling the condenser 171 may be
provided.
The pressure of the refrigerant in the liquid phase condensed by
the condenser 171 is lowered by the expansion valves 181 and 182.
That is, the expansion valves 181 and 182 decompress the
refrigerant in the high-pressure and liquid phase to pressure at
which the refrigerant may be evaporated by throttling. Throttling
refers to a phenomenon that when a fluid passes through a narrow
path, such as a nozzle or an orifice, the pressure of the fluid is
lowered even without heat exchange with the outside.
Further, the expansion valves 181 and 182 adjust the amounts of the
refrigerant so that the refrigerant may absorb sufficient thermal
energy from the evaporators 191 and 192. Particularly, if
electronic expansion valves are used as the expansion valves 181
and 182, opening/closing and opening degrees of the expansion
valves 181 and 182 are adjusted by a driving unit (driver) 220
under the control of a control unit (controller) 210 which will be
described later.
The evaporators 191 and 192 are provided in the ducts (not shown)
in the inner space of the main body 110, as described above, and
evaporate the refrigerant in the low-pressure and liquid phase
decompressed by the expansion valves 181 and 182.
During such an evaporating process, the refrigerant absorbs latent
heat from the evaporators 191 and 192, and the evaporators 191 and
192 discharging thermal energy cool air around the evaporators 191
and 192.
The refrigerant in the low-pressure and vapor phase evaporated by
the evaporators 191 and 192 is provided back to the compressor 161,
thereby repeating the refrigerating cycle.
The compressor 161 is installed in the machine chamber (not shown)
provided in the lower portion of the main body 110, compresses the
refrigerant in the low-pressure and vapor phase evaporated by the
evaporators 191 and 192 using rotating force of a motor, and
transfers the compressed refrigerant to the condenser 171 under
high pressure. The refrigerant circulates along the condenser 171,
the expansion valves 181 and 182 and the evaporators 191 and 192
due to pressure generated from the compressor 161.
The compressor 161 of the refrigerator 100 in accordance with an
embodiment employs a three-phase brushless direct current (BLDC)
motor. However, embodiments are not limited thereto, and the
compressor 161 may employ an inductive AC servomotor or a
synchronous AC servomotor.
Rotating force generated by the three-phase BLDC motor is converted
into translating force by a piston of the compressor 161, and the
piston compresses the refrigerant in the low-pressure and vapor
phase provided from the evaporators 191 and 192 into a
high-pressure state through the translating force.
Otherwise, rotating force generated by the three-phase BLDC motor
may be transmitted to rotary blades connected to a rotor of the
three-phase BLDC motor, and the refrigerant in the low-pressure and
vapor phase may be compressed by stick-slip between the rotary
blades and a container of the compressor 161.
Hereinafter, with reference to FIG. 2 briefly illustrating the
structure of the three-phase BLDC motor in accordance with
embodiment, the motor of the compressor 161 employing the
three-phase BLDC motor will be described.
As shown in FIG. 2, the three-phase BLDC motor uses coils as a
stator and uses a permanent magnet as a rotor, and varies current
flowing in the stator through a switching circuit, such as an
inverter, so as to continuously rotate the rotor, without a brush
to vary current flowing in the coils.
Further, designated ends of three coils are connected (COM), and
the other ends of the coils form input terminals In1, In2 and In3
of the three-phase BLDC motor.
Concretely, if current flows from the input terminal In1 to the
input terminal In3 of the three-phase BLDC motor while the rotor is
rotated in the clockwise direction, the side of the coil Da at the
inside of the stator becomes the south pole S and the side of the
coil L3b at the inside of the stator becomes the south pole S based
on Ampere's right-handed screw rule. Thereby, the south pole S of
the coil L3b attracts the north pole N of the permanent magnet used
as the rotor, and thus, the permanent magnet used as the rotor may
be rotated in the clockwise direction.
Thereafter, if current flows from the input terminal In2 to the
input terminal In3 of the three-phase BLDC motor, the side of the
coil L2a at the inside of the stator becomes the south pole S and
the side of the coil L3b at the inside of the stator becomes the
south pole S. Thereby, the south pole S of the coil L2a attracts
the north pole N of the permanent magnet used as the rotor, and
thus, the permanent magnet used as the rotor may be continuously
rotated in the clockwise direction.
The BLDC motor may vary current flowing to the coils used as the
stator in the above-described manner, and thus continuously rotate
the permanent magnet used as the rotor.
In order to continuously rotate the BLDC motor, as described above,
current flowing in the coils used as the stator needs to be
properly varied according to the position of the rotor. For this
purpose, the position of the rotor is sensed using a hall sensor or
an encoder.
However, in high-temperature and high-pressure environments, such
as the compressor 161 of the refrigerator 100, installation of a
hall sensor may be difficult. Therefore, without use of a hall
sensor, the position of the rotor is detected by measuring back
electromotive force generated due to rotation of the permanent
magnet used as the rotor.
Hereinafter, with reference to FIG. 3 illustrating back
electromotive force generated from each coil of the three-phase
BLDC motor in accordance with an embodiment and FIGS. 4A and 4B
illustrating back electromotive force generated from each coil and
current flowing in each coil of the three-phase BLDC motor in
accordance with an embodiment, current flowing in each coil of the
three-phase BLDC motor in accordance with an embodiment will be
described.
As a representative method to measure the position of the rotor
through measurement of back electromotive force without use of a
hall sensor, a zero crossing point of back electromotive force is
used.
If the north pole N of the permanent magnet passes through the coil
L1a, back electromotive force of the coil Da increases based on
Lenz's law when the north pole N of the permanent magnet is close
to the coil Da. That is, back electromotive force of the coil Da
becomes the maximum when the north pole N of the permanent magnet
is closest to the coil L1a, and decreases when the north pole N of
the permanent magnet becomes distant from the coil L1a.
Since the permanent magnet used as the rotor is a dipole having the
N pole and the S pole and is rotated, as the N pole of the
permanent magnet becomes distant from a coil, the S pole of the
permanent magnet becomes close to the coil. When the S pole of the
permanent magnet is close to the coil L1a, back electromotive force
of the coil Da further decreases and thus reaches negative value
via `0`.
Thus, back electromotive force of the coil L1 is varied according
to rotation of the permanent magnet used as the rotor.
With reference to FIG. 3, zero crossing of the coil L1 in which
back electromotive force of the coil L1 becomes `0` according to
rotation of the permanent magnet used as the rotor is generated
when the rotating angle of the rotor becomes 90.degree. and when
the rotating angle of the rotor becomes 270.degree., zero crossing
of the coil L2 is generated when the rotating angle of the rotor
becomes 30.degree. and when the rotating angle of the rotor becomes
210.degree., and zero crossing of the coil L3 is generated when the
rotating angle of the rotor becomes 150.degree. and when the
rotating angle of the rotor becomes 330.degree..
When current flowing in each coil and back electromotive force
generated by rotation of the rotor have the same phase, the maximum
magnetic torque occurs. That is, if current I1 flowing in the coil
L1, current I2 flowing in the coil L2 and current I3 flowing in the
coil L3 flow when back electromotive force of each coil is
constant, as shown in FIG. 4A, the magnetic torque of the BLDC
motor becomes the maximum. FIG. 4B briefly illustrates the
direction of current flowing in each coil.
With reference to FIG. 4A, if the rotor is further rotated by an
angle of 30.degree. after back electromotive force of each coil
becomes `0`, when current flows in the coils, the magnetic torque
of the BLDC motor may become the maximum. That is, if the rotor is
further rotated by an angle of 30.degree. after generation of zero
crossing of back electromotive force, the maximum magnetic torque
may be acquired by converting the phase of driving current.
Such operation may be executed by allowing a compressor driving
unit 260 to control current flowing in each input terminal of the
three-phase BLDC motor under the control of the control unit 210
which will be described later.
FIG. 5 is a block diagram illustrating control flow of the
refrigerator 100 in accordance with an embodiment. Now, with
reference to FIG. 5, control flow of the refrigerator 100 in
accordance with an embodiment will be described.
Target temperatures to cool the storage chambers 121 and 122 of the
refrigerator 100 so as to store articles for a long time are set.
The initial values of the target temperatures are set when the
refrigerator 100 is manufactured, and then the target temperatures
may be varied by operation of a user. In general, the target
temperature of the freezing chamber 121 is set to -20.degree. C. as
an initial value, and the target temperature of the refrigerating
chamber 122 is set to 4.degree. C. as an initial value.
The upper and lower limits to maintain the set target temperatures
of the refrigerator 100 are set. That is, when the temperatures of
the storage chambers 121 and 122 are increased to above the upper
limits, the refrigerator 100 starts operation and cools the storage
chambers 121 and 122, and when the temperatures of the storage
chambers 121 and 122 are decreased to below the lower limits, the
refrigerator 100 stops operation. In general, the upper limit is
set to be higher than the target temperature by 1.degree. C., and
the lower limit is set to be lower than the target temperature by
1.degree. C.
The refrigerator 100 in accordance with an embodiment is operated
in a general operation mode or a power-saving operation mode
according to user selection or a sensing result of the temperature
sensing units 141, 142 and 143. In more detail, the refrigerator
100 may be operated in the general operation mode as a basic
operation mode, and be operated in the power-saving operation mode
by selecting the power-saving operation mode by a user through the
input unit 111 which will be described later, or a sensing result
of the external temperature sensing unit 143. For example, if the
external temperature is lower than the target temperature of the
refrigerating chamber 122, the refrigerator 100 may be operated in
the power-saving operation mode.
Further, a driving mode of the compressor 161 is determined
according to the operation mode of the refrigerator 100. That is,
when the refrigerator 100 is operated in the general operation
mode, the compressor 161 is driven in a general driving mode, and
when the refrigerator 100 is operated in the power-saving operation
mode, the compressor 161 is driven in a low-speed driving mode.
The input unit 111 may employ a button switch, a membrane switch or
a touchscreen. The input unit 111 receives instructions regarding
operation of the refrigerator 100, such as whether or not power is
supplied to the refrigerator 100, the target temperatures of the
freezing chamber 121 and the refrigerating chamber 132, and whether
or not the power-saving operation mode is selected, from a
user.
The display unit 112 may employ a liquid crystal display (LCD)
panel or an organic light emitting diode (OLED) panel. The display
unit 112 displays information regarding operation of the
refrigerator 100, such as the target temperatures and current
temperatures of the freezing chamber 121 and the refrigerating
chamber 132 and whether or not the power-saving operation mode is
selected.
A storage unit 240 may employ a flash memory. The storage unit 240
stores various pieces of information regarding operation of the
refrigerator 100, such as the target temperatures of the freezing
chamber 121 and the refrigerating chamber 122, the general
operation mode and the power-saving operation mode.
The driving unit 220 includes a cooling fan driving unit 250
driving the cooling fans 151 and 152, an expansion valve driving
unit 280 driving the expansion valves 181 and 182, and a compressor
driving unit 260 driving the compressor 161.
The cooling fan driving unit 250 drives cooling fan motors (not
shown) to rotate the cooling fans 151 and 152 under the control of
the control unit 210, and the expansion valve driving unit 280
drives solenoids of the expansion valves 181 and 182 to open or
close the expansion valves 181 and 182 under the control of the
control unit 210.
A compressor motor driving apparatus 200 including the compressor
driving unit 260, a position sensing unit (position sensor) 230 and
the control unit 210 selects a driving mode of the compressor 161
and drives the compressor 161 according to the selected driving
mode.
Hereinafter, with reference to FIG. 6 illustrating a control flow
of the compressor motor driving apparatus 200, the compressor motor
driving apparatus 200 will be described.
The compressor motor driving apparatus 200 includes the compressor
driving unit 260 driving the compressor 161, the position sensing
unit 230 sensing the position of the rotor of a compressor motor
162, and the control unit 210 controlling torque and rotating speed
of the compressor motor 162.
Now, with reference to FIG. 7 briefly illustrating the compressor
driving unit 260 and the position sensing unit 230, the compressor
driving unit 260 and the position sensing unit 230 will be
described.
The compressor driving unit 260 includes an external power source
1, a rectifier circuit 262, a smoothing circuit 264, and a driving
circuit 266.
The external power source 1 is a commercial AC power source having
a frequency of 50 Hz or 60 Hz.
The rectifier circuit 262 connects four diodes D11, D12, D13 and
D14 using bridges and converts a negative value of AC voltage of
the external power source 1 into a positive value, thus generating
voltage of an English letter M shape.
The smoothing circuit 264 includes one capacitor, and converts the
voltage of the English letter M shape output from the rectifier
circuit 262 into DC voltage having a constant value. Constant power
voltage is applied to the driving circuit 266 which will be
described later, by the smoothing circuit 264.
The driving circuit 266 is an inverter including six switches Q1a,
Q1b, Q2a, Q2b, Q3a and Q3b.
In the driving circuit 266, a total of three pairs of switches,
each pair of switches of which is connected in series between power
and ground, is provided.
The six switches Q1a, Q1b, Q2a, Q2b, Q3a and Q3b may be
metal-oxide-silicon field effect transistors (MOSFETs) or bipolar
junction transistors (BJTs). Further, six control signals from the
control unit 210 which will be described later are input to gates
or bases of the six MOSFETs or BJTs.
In the driving circuit 266, two switches which are not located on
the same row among the six switches Q1a, Q1b, Q2a, Q2b, Q3a and Q3b
are turned on by the control signals from the control unit 210, and
thus provide driving current to the three-phase BLDC motor.
Concretely, when the switches Q1a and Q3b are turned on, the
driving circuit 266 provides driving current to the coils L1 and L3
of the three-phase BLDC motor, when the switches Q2a and Q3b are
turned on, the driving circuit 266 provides driving current to the
coils L2 and L3 of the three-phase BLDC motor, and when the
switches Q2a and Q1b are turned on, the driving circuit 266
provides driving current to the coils L2 and L1 of the three-phase
BLDC motor. In the same manner, when the switches Q3a and Q1b are
turned on, the driving circuit 266 provides driving current to the
coils L3 and L1 of the three-phase BLDC motor, when the switches
Q3a and Q2b are turned on, the driving circuit 266 provides driving
current to the coils L3 and L2 of the three-phase BLDC motor, and
when the switches Q1a and Q2b are turned on, the driving circuit
266 provides driving current to the coils L1 and L2 of the
three-phase BLDC motor.
It causes driving current of the same type as current to generate a
rotating magnetic field rotating the rotor of the above-described
three-phase BLDC motor to flow in each coil of the compressor
motor.
The position sensing unit 230 has the form of a voltage divider in
which two resistors are connected in series between the three input
terminals of the compressor motor and ground. That is, resistors
R1a and R1b are provided between the input terminal In1 of the
compressor motor and ground, resistors R2a and R2b are provided
between the input terminal In2 of the compressor motor and ground,
and resistors R3a and R3b are provided between the input terminal
In3 of the compressor motor and ground. Further, the position
sensing unit 230 provides voltage of a node to which a pair of
resistors is connected to the control unit 210.
When the ratio of the resistors R1a, R2a and R3a connected to the
input terminals In1, In2 and In3 of the compressor motor to the
resistors R1b, R2b and R3b connected to ground is set to 99:1, the
voltage divider outputs voltage of an intensity of 1/100 of input
voltage. That is, when back electromotive force of 310V is
generated from the input terminal In1, only 3.1V is provided to the
control unit 210.
Based on driving of the above-described three-phase BLDC motor,
when driving current does not flow in the coil of the three-phase
BLDC motor, zero crossing in which back electromotive force of the
coil becomes `0` is generated. Therefore, a zero crossing point may
be detected by measuring voltage of the input terminal connected to
the coil to which driving current is not applied among the three
input terminals of the three-phase BLDC motor. Concretely, current
flows in a pair of resistors forming the voltage divider by back
electromotive force generated from the input terminal in which
driving current does not flow. That is, when back electromotive
force becomes `0`, the output of the voltage divider becomes `0`.
Such a point of time when the output of the voltage divider becomes
`0` may be judged as generation of zero crossing, and the position
of the rotor may be estimated using the point of time.
For example, when the switches Q1a and Q3b of the driving circuit
266 are turned on and thus current flows in the coils L1 and L2, it
is estimated that zero crossing is generated from the coil L2,
current flows in the resistors R2a and R2b of the position sensing
unit 230 by back electromotive force generated from the coil L2,
and voltage of the node between the resistors R2a and R2b becomes
`0` when zero crossing in which back electromotive force becomes
`0` is generated. Further, as described above, when the rotor is
further rotated by an angle of 30.degree. after generation of zero
crossing, driving current may flow in the coils L2 and L3 by
turning the switches Q2a and Q3b on and thus a rotating magnetic
field may be formed.
The control unit 210 judges whether or not the cooling device 161,
171, 181, 182, 191 and 192 is driven based on the sensing result of
the first and second temperature sensing units 141 and 142 provided
in the storage chambers 121 and 122, and selects the operation mode
of the refrigerator 100 based on user instructions or the sensing
result of the external temperature sensing unit 143.
Concretely, the control unit 210 controls the driving unit 220 to
drive the cooling device 161, 171, 181, 182, 191 and 192 when the
temperatures of the storage chambers 121 and 122 reach the upper
limits, and controls the driving unit 220 to stop driving of the
cooling device 161, 171, 181, 182, 191 and 192 when the
temperatures of the storage chambers 121 and 122 reach the lower
limits, based on the sensing result of the first temperature
sensing unit 141 sensing the temperature of the freezing chamber
121 and the second temperature sensing unit 142 sensing the
temperature of the refrigerating chamber 122.
Further, the control unit 210 switches the current operation mode
of the refrigerator 100 to the power-saving operation mode when the
external temperature is lower than the target temperature of the
refrigerating chamber 122 and switches the current operation mode
of the refrigerator 100 to the general operation mode when the
external temperature is higher than the target temperature of the
refrigerating chamber 122, based on the sensing result of the
external temperature sensing unit 143 sensing the temperature of
the outside of the refrigerator 100. Moreover, the control unit 210
switches the current operation mode of the refrigerator 100 to the
power-saving operation mode when the power-saving operation mode is
selected by a user through the above-described input unit 111.
Further, the control unit 210 controls the driving unit 220 to
drive the cooling device 161, 171, 181, 182, 191 and 192 in the
general driving mode when the refrigerator 100 is operated in the
general operation mode, and controls the driving unit 220 to drive
the cooling device 161, 171, 181, 182, 191 and 192 in the low-speed
driving mode when the refrigerator 100 is operated in the
power-saving operation mode.
With reference to FIG. 8 illustrating control flow of the control
unit 210 regarding control of the compressor 161 and selection of
the driving mode of the compressor 161, the control unit 210
includes a mode selector 212, a speed controller 214, a driving
signal generator 216 and a pulse width modulator 218.
The mode selector 212 selects the operation mode of the
refrigerator 100 based on mode selection of a user input through
the input unit 111 or the temperature sensing result of the
external temperature sensing unit 142, generates a mode control
signal according to the selected operation mode, and provides the
generated mode control signal to the driving signal generator 216.
The driving mode of the compressor 161 and the conduction method of
the compressor motor 162 are determined according to the selected
operation mode of the refrigerator 100.
Concretely, if power-saving operation is selected by a user or the
external temperature is lower than the target temperature of the
refrigerating chamber 122 as the sensing result of the external
temperature sensing unit 143, the refrigerator 100 is operated in
the power-saving operation mode, the compressor 161 is driven in
the low-speed driving mode, and the compressor motor 162 is driven
in a 90 degree conduction method. Further, if power-saving
operation is released by a user or the external temperature is
higher than the target temperature of the refrigerating chamber 122
as the sensing result of the external temperature sensing unit 143,
the refrigerator 100 is operated in the general operation mode, the
compressor 161 is driven in the general driving mode, and the
compressor motor 162 is driven in a 120 degree conduction
method.
The speed controller 214 generates a speed control signal
controlling the rotating speed of the compressor motor 162 and a
rotor position signal based on the sensing result of the position
sensing unit 230, and provides the speed control signal and the
rotor position signal to the driving signal generator 216.
Concretely, the speed controller 214 calculates the position of the
rotor based on zero crossing points sensed by the position sensing
unit 230 at which back electromotive force of the three coils of
the compressor motor 162 becomes `0`, and generates a rotor
position signal based on the calculated position of the rotor.
Further, the speed controller 214 calculates the rotating speed of
the rotor, i.e., the speed of the motor, by differentiating the
calculated position of the robot by time, and generates a speed
control signal by comparing the calculated speed of the motor with
a target speed of the motor to normally operate the compressor
161.
The driving signal generator 216 generates a driving signal to
rotate the compressor motor 162 based on the mode control signal of
the mode selector 212 and the rotor position signal of the speed
controller 214 when the temperature of the freezing chamber 121
sensed by the first temperature sensing unit 141 is higher than the
upper limit of the freezing chamber 121, i.e., -19.degree. C. or
when the temperature of the refrigerating chamber 122 sensed by the
second temperature sensing unit 142 is higher than the upper limit
of the refrigerating chamber 122, i.e., 5.degree. C.
Concretely, the driving signal generator 216 generates driving
signals to turn the six switches Q1a, Q1b, Q2a, Q2b, Q3a and Q3b of
the compressor driving unit 260 on/off according to the position of
the rotor, in order to generate a rotating magnetic field on the
compressor motor 162, Here, the driving signal generator 216
generates driving signals to turn the six switches Q1a, Q1b, Q2a,
Q2b, Q3a and Q3b of the compressor driving unit 260 on/off so as to
drive the compressor motor 162 in the 120 degree conduction method,
when the mode selector 212 selects the general driving mode, and
generates driving signals to turn the six switches Q1a, Q1b, Q2a,
Q2b, Q3a and Q3b of the compressor driving unit 260 on/off so as to
drive the compressor motor 162 in the 90 degree conduction method,
when the mode selector 212 selects the low-speed driving mode. The
120 degree conduction method and the 90 degree conduction method of
the compressor motor 162 will be described later.
Further, the driving signal generator 216 generates a pulse width
control signal based on the mode control signal of the mode
selector 212 and the speed control signal of the speed controller
214. The pulse width control signal controls the pulse width of
driving current, if the driving current of a pulse type is applied
to control the speed of the compressor motor 162.
Concretely, the driving signal generator 216 generates a pulse
width control signal to decrease the pulse width of driving current
so as to decrease torque of the motor when the speed of the motor
calculated by the speed controller 214 is higher than a target
speed, and generates a pulse width control signal to increase the
pulse width of driving current so as to increase torque of the
motor when the speed of the motor is lower than the target
speed.
Now, with reference to FIG. 9 illustrating control flow of the
pulse width modulator 218 and FIG. 10 illustrating outputs of the
respective components of the pulse width modulator 218, the pulse
width modulator 218 will be described.
The pulse width modulator 218 includes a digital-to-analog
converter (DAC) 218a, a triangular wave generator 218b, a
comparator 218c and an AND gate 218d.
The DAC 218a converts the pulse width control signal provided by
the driving signal generator 216 into an analog value, i.e., a
pulse width.
The triangular wave generator 218b generates triangular waves of a
predetermined frequency. The triangular waves generated by the
triangular wave generator 218b have an isosceles triangular shape
rather than a wedge shape.
The comparator 218c compares the output of the DAC 218a with the
output of the triangular wave generator 218b. The output of the DAC
218a is input to a positive (+) input terminal of the comparator
218c, and the output of the triangular wave generator 218b is input
to a negative (-) input terminal of the comparator 218c.
Concretely, when the output of the DAC 218a is greater than the
output of the triangular wave generator 218b, the comparator 218c
outputs high voltage, i.e., power voltage Vcc, and when the output
of the DAC 218a is smaller than the output of the triangular wave
generator 218b, the comparator 218c outputs low voltage, i.e.,
0V.
With reference to FIG. 10, the output of the triangular wave
generator 218b increases from 0 to t2, and decreases from t2 to t4.
Further, the output of the DAC 218a is greater than the output of
the triangular wave generator 218b from 0 to t1 and thus the
comparator 218c outputs power voltage in this section, and the
output of the DAC 218a is smaller than the output of the triangular
wave generator 218b from t1 to t3 and thus the comparator 218c
outputs 0V in this section.
Now, pulse width modulating operation will be described. When the
pulse width is increased by the pulse width control signal provided
by the driving signal generator 216, the output of the DAC 218a is
increased for a longer time than the output of the triangular wave
generator 218b and thus, the comparator 218c outputs square waves
having a wide pulse width, and when the pulse width is decreased by
the pulse width control signal provided by the driving signal
generator 216, the output of the DAC 218a is decreased for a longer
time than the output of the triangular wave generator 218b and
thus, the comparator 218c outputs square waves having a narrow
pulse width.
The AND gate 218d executes AND operation of the driving signal
which is the target object of pulse width modulation and the output
of the comparator 218c. Concretely, when the driving signal is
`high`, i.e., the power voltage, a signal, the pulse width of which
has been modulated, is output, and when the driving signal is
`low`, i.e., 0V, the low driving signal is output, as it is.
Here, six pulse width modulators 218 corresponding to six driving
signals of the driving signal generator 216 may be provided, or one
pulse width modulator 218 may be provided and thus modulate the
pulse width of only a driving signal requiring pulse width
modulation among the six driving signals.
FIG. 11 is a view illustrating flow of driving signals and driving
current driving the compressor motor if the refrigerator in
accordance with an embodiment is in the general operation mode.
When a user releases the power-saving operation mode or the
external temperature of the refrigerator 100 is higher than the
target temperature of the refrigerating chamber 122, the
refrigerator 100 is operated in the general operation mode. If the
refrigerator 100 is operated in the general operation mode, the
driving unit 220 is driven in the general driving mode, and the
compressor motor 162 is driven in the 120 degree conduction
method.
When one of the six switches Q1a, Q1b, Q2a, Q2b, Q3a and Q3b of the
compressor driving unit 260 is turned on, the on state of the
switch is maintained until the rotor is further rotated by an angle
of 120.degree., and thus such a method is called the 120 degree
conduction method.
Further, when another of six switches Q1a, Q1b, Q2a, Q2b, Q3a and
Q3b is turned on, the on state of the switch is continuously
maintained until the rotor is rotated by an angle of 60.degree.,
but the on and off states of the switch are repeated by the driving
signal, the pulse width of which has been modulated, until the
rotor is rotated by an angle of 120.degree. after rotation of the
robot by an angle of angle of 60.degree..
Concretely, until the rotor is rotated by an angle of 60.degree.,
the switch Q1a is turned on while modulating the pulse width and
the switch Q3b is continuously turned on. Thereafter, until the
rotor is rotated by an angle of 120.degree., the switch Q2a is
continuously turned on and the switch Q3b is turned on while
modulating the pulse width. Thereafter, until the rotor is rotated
by an angle of 180.degree., the switch Q2a is turned on while
modulating the pulse width and the switch Q1b is continuously
turned on.
Thereafter, two switches are turned on in such a manner. That is,
one switch is continuously turned on and another switch is turned
on while modulating the pulse width.
FIG. 12 is a view illustrating flow of driving signals and driving
current driving the compressor motor if the refrigerator in
accordance with an embodiment is in the power-saving operation
mode.
When a user selects the power-saving operation mode or the external
temperature of the refrigerator 100 is lower than the target
temperature of the refrigerating chamber 122, the refrigerator 100
is operated in the power-saving operation mode. If the refrigerator
100 is operated in the power-saving operation mode, the driving
unit 220 is driven in the low-speed driving mode, and the
compressor motor 162 is driven in the 90 degree conduction
method.
When one of the six switches Q1a, Q1b, Q2a, Q2b, Q3a and Q3b of the
compressor driving unit 260 is turned on, the on state of the
switch is maintained until the rotor is further rotated by an angle
of 90.degree., and thus such a method is called the 90 degree
conduction method.
Concretely, a point of time when the switch is turned on according
to the position of the rotor is the same as in the above-described
120 degree conduction method, but a point of time when the switch
is turned off is faster than in the above-described 120 degree
conduction method by an angle of 30.degree.. Therefore, a time of
turning the switch on while modulating the pulse width is half the
time in the 120 degree conduction method, and thus, in order to
acquire the same rotating speed as in the 120 degree conduction
method, the pulse width becomes twice the pulse width in the 120
degree conduction method and the number of switching to modulate
the pulse width becomes half the number of switching in the 120
degree conduction method.
Until the rotor is rotated by an angle of 30.degree., the switch
Q1a is turned on while modulating the pulse width and the switch
Q3b is continuously turned on.
Thereafter, until the rotor is rotated by an angle of 60.degree.,
the switch Q1a is turned off and only the switch Q3b is turned on.
Even if the switch Q1a is turned off, current flowing in the coils
L1 and L3 does not rapidly disappear but freewheeling current flows
in the coils L1 and L3 for a designated time due to inductances of
the coils L1 and L3. Such freewheeling current flows in a direction
from the coil L1 to the coil L3 in the same manner as when both the
switches Q1a and Q3b are turned on, current flowing from the coil
L3 passes through a freewheeling diode D1b connected to the switch
Q3b and the switch Q1b in parallel and enters the coil L1.
Thereafter, until the rotor is rotated by an angle of 90.degree.,
the switch Q2a is continuously turned on and the switch Q3b is
turned on while modulating the pulse width.
Thereafter, until the rotor is rotated by an angle of 120.degree.,
the switch Q3b is turned off and only the switch Q2a is turned on.
Even if the switch Q3b is turned off, freewheeling current is
generated due to inductances of the coils L2 and L3.
Thereafter, the control unit 210 controls the six switches Q1a,
Q1b, Q2a, Q2b, Q3a and Q3b of the compressor driving unit 260 in
the above-described method, thus driving the compressor motor 162
in the 90 degree conduction method.
FIG. 13 is a view illustrating efficiency of the compressor motor
162 if the refrigerator 100 in accordance with an embodiment is in
the power-saving operation mode and efficiency of a compressor
motor of a conventional refrigerator.
If the compressor motor 162 is rotated at a low speed, when the
compressor motor 162 in accordance with an embodiment is driven in
the 90 degree conduction method, the modulated pulse width of
driving current is further increased and the number of switching to
modulate the pulse width is reduced.
Therefore, as shown in FIG. 13, if the refrigerator 100 is operated
in the power-saving operation mode and the compressor motor 162 is
rotated at a low speed, the 90 degree conduction method exhibits
higher efficiency than the conventional 120 degree conduction
method.
As is apparent from the above description, a BLDC motor driving
apparatus and a refrigerator using the same in accordance with an
embodiment increase a pulse duty of driving current by converting a
conduction method of the driving current so as to drive a BLDC
motor at a low speed when the refrigerator is operated in a
power-saving operation mode.
Although a few embodiments have been shown and described, it would
be appreciated by those skilled in the art that changes may be made
in these embodiments without departing from the principles and
spirit of the disclosure, the scope of which is defined in the
claims and their equivalents.
* * * * *